Intracellular proteases from rat liver will be purified, characterized, and tested for their ability to degrade various enzymes. The intracellular localization of these enzymes will be studied by conventional cellular fractionation methods and by electron microscopy, using protease substrates whose end products can be converted to electron-dense derivatives. The pathway of secretion of lysosomal enzymes in Tetrahymena will also be studied, using substrates that permit visualization of enzymes by electron microscopy, as well as through the use of ferritin-labelled antibodies.